It's so sad to read this. My youngest sister got in touch with me when she was sixteen and I was already a couple of years DF'd. She didn't want the religion anymore, she was struggling with gay feelings and secretly celebrated her birthday with her schoolfriends in the shopping mall... I was happy that she confided in me, but I knew it could change overnight, which it did. I got angry with my parents about them trying to keep me away from my sister and wrote about it on my blog. All of a sudden I was bad, evil etc. Not long after that she decided to become a witness after all and did all she could to 'hook' an elder's son, in which she succeeded, and got baptized and married and is now playing the perfect JW-housewife, mother of two and from what I know she hates my guts. Haven't heard from her in a long time and actually I don't care anymore after the things she did to try to hurt and harm me on the internet.
With a lot of JW-born in teenagers it's soooo unpredictable which way they will turn. One moment they hate being a witness and before you know it they're baptized and hooked by the cult. Creepy and terribly sad.
In your case I would try what the others already suggested. Write her that you will love her unconditionally, that you will always be her loving sister and that there is no need to cut all ties.